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Neutrino Flavor Evolution in Neutron Star Mergers [HEAP]

We examine the flavor evolution of neutrinos emitted from the disk-like remnant (hereafter called \lq\lq neutrino disk\rq\rq) of a binary neutron star (BNS) merger. We specifically look at the neutrinos emitted from the center of the disk, along the polar axis perpendicular to the equatorial plane. In order to better understand the underlying physics and the nature of the ensuing flavor evolution, we carried out two-flavor simulations using a variety of different possible initial neutrino luminosities and energy spectra, and for comparison, also a three-flavor simulation for one of the cases (the bipolar spectral swap). The flavor evolution was found to be highly dependent on the initial neutrino luminosities and energy spectra; in particular, we found two broad classes of results depending on the sign of the initial net electron neutrino lepton number (i.e., the number of neutrinos minus the number of antineutrinos). In the antineutrino dominated case, we found that the Matter-Neutrino Resonance (MNR) effect dominates, consistent with previous literature, whereas in the neutrino dominated case, a bipolar spectral swap develops. For the latter case, in addition to the swap at low energies, a particularly interesting feature of our results was the development of a high energy electron neutrino tail. We argue that the high energy electron neutrinos in the tail of the distribution may have implications for the electron fraction of the material they interact with, and could thereby influence the $r$-process in BNS merger environments.